Integrated circuits contain a plurality of highly integrated circuit elements that are connected via complex metal plating, for example transistors, resistors, or capacitors. During operation of the integrated circuits, currents flow across the metal plating; due to the small line cross sections, i.e., within connecting points of the metal levels, very high current densities, typically in the range of up to 1 MA/cm2, may occur. This results in a failure mechanism of so-called electromigration. Electrons collide with atoms of the strip conductor, and a pulse transfer takes place that results in a displacement or diffusion of the metal atom in the direction of the electron flow. If this results in a mass flow divergence, cavities form in the strip conductor; i.e., at certain locations in the metal plating, more metal atoms move away than are delivered, for example at grain boundaries or vias, which are made of a different material such as tungsten. It is disadvantageous that the strip conductor breaks off; i.e., the strip conductor is interrupted and the electrical functionality of the ASIC is no longer present. Rising temperature likewise increases the effect of the electromigration. The effect is also significantly dependent on the intrinsic heating of the strip conductor, the signal pattern of the load (with AC currents generating less degradation), the metal used, such as Al, W, Cu, Ti, etc., and its mechanical properties, and the processing parameters of the metal plating, including process errors, and the layout, for example the length and width of the strip conductor.
For determining failure mechanisms, it is known to evaluate the maximum load capacity of the strip conductors, vias, and contacts. Methods and test structures for this purpose are standardized, for example JEDEC standards JESD61, JESD63, and JESD87. Furthermore, simulation of the effect of the electromigration, using the finite elements method, is known. In addition, accelerated test methods and reliability tests are correlated with the conditions of the actual application, for example as described in “Simulation of Electromigration Test Structures with and without Extrusion Monitors,” V. Hein, ICSE 2006.
The document DE 10 2008 000 218 A1 discusses an improved test structure having the objective of reducing the corresponding testing times in quick tests.
It is likewise believed to be understood that the design of the strip conductor widths is based on assumptions concerning the actual field load and load capacity of the metal plating. If the actual load is higher or the metal plating is defective, there is the risk of failures despite having passed quality inspection and function testing.
The document U.S. Pat. No. 8,890,556 B2 discusses continuous monitoring of the electromigration in the application, as well as corresponding measuring cycles that are carried out in the ASIC.
Structures from standard testing are based on long-term measurements of individual strip conductors, and at the time of testing detect the most critical dimension of the strip conductor for which the strongest electromigration is present, in order to establish the maximum load capacity. The objective of the qualification is not relative measurement, since such measuring devices may indicate absolute values for the resistance, and absolute values must be indicated for specifying the maximum load capacity.
The qualification tests only a limited quantity of material. Protection is to be provided for the most critical load that occurs in the field, i.e., the maximum current on the most unstable strip conductor at the highest temperature. This results in oversizing; however, unexpected low load capacities, for example due to process fluctuations, or unplanned overloads, for example continuous operation at the load limit, cannot be addressed in a cost-effective manner.
The electromigration is strongly dependent on the strip conductor width, and has the “bamboo effect.” Here, the electromigration stability of very thin strip conductors is very high, since statistically, grain boundaries are very often perpendicular to the strip conductor; however, a grain boundary hinders the migration of the metal atoms. In addition, thin strip conductors are well stabilized mechanically by the surrounding dielectrics. In turn, very wide strip conductors provide the current flow with sufficient alternatives on parallel paths in the event of localized degradation.
In this regard, there is an average width, typically in the range of the average grain size, for example 1 μm-3 μm, which represents electromigration that is particularly critical for the failure mechanism. This effect has not been considered thus far in integrated test structures.
The known electromigration structures measured in situ test only one layout variant, and compare its resistance to an unstressed reference element or initial value.
Even if it were conceivable to integrate the structures, mentioned in the document U.S. Pat. No. 8,890,556 B2, multiple times with modified metal plating variants, the space requirements would indeed be disadvantageous.